Yes, relationships between coaches/professors and students are unethical. But the coaches/professors who ARE ethical WILL wait until graduation or until the student leaves the school to embark on a relationship.

it happens a LOT. The only thing unethical is if it starts beforehand. But wanting it to start beforehand, and starting it beforehand are two completely different things.

I actually know quite a few professors married to former students. Its just happens but it doesn't mean anyone acted on it before hand.